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How do you type an async function return in TypeScript?
This tests if you know async functions always return Promise<T>. A great answer: annotate Promise<User>, return User from the body, and note TypeScript implicitly wraps it. A red flag is omitting Promise and typing the return as just User.
How would you prevent search-as-you-type race conditions with AbortController?
Abort the previous request before each new keystroke, pass the fresh signal into fetch, and ignore the AbortError.
Implement inSequence(tasks) to execute promise-returning functions sequentially
Tests async/await control flow versus Promise.all parallelism. A strong answer uses a for-of loop with await inside an async function, accumulates results in order, and stops cleanly on rejection. Red flag: suggesting Promise.all or forEach with await.
Write a fetch call and log JSON from the Response
This tests async HTTP literacy and fetch's two-stage promise resolution. A strong answer awaits fetch, then awaits response.json(), logs result, and notes fetch does not throw on 4xx/5xx.
How would you modify fetch to handle HTTP error statuses?
Verify response.ok in then and throw if false, then catch network failures separately.
Write a createUser function that POSTs JSON via fetch
Precise fetch configuration for JSON POST requests. A strong answer names method POST, headers Content-Type application/json, and body JSON.stringify(data) with return typing. Red flag: passing the raw object as body or omitting headers.
Write a typed async fetchUser with error handling
Tests promise-based fetch plus TypeScript return-type contracts. A strong answer checks response.ok before response.json() and types the return as Promise<User>. A red flag is swallowing 4xx/5xx errors silently.
How do you include a JWT in a fetch request?
Tests knowledge of the fetch options object and Bearer scheme syntax. A strong answer sets headers: { Authorization: Bearer <token> } as the second argument and notes fetch does not auto-attach tokens. Red flag: omitting Bearer or hardcoding secrets.
How do you handle an API returning 200 for success and failure?
This tests TypeScript narrowing and runtime validation of ambiguous 200 responses. A strong answer uses a discriminated union, narrows with a type guard, and validates shape before branching. Red flag: casting the body with as and skipping runtime checks.
Create a generic fetchJson wrapper with typed response and error handling
Generic T parameter, optional RequestInit, throw if !res.ok, return res.json() as Promise<T>.
How do you cancel pending fetches using AbortController?
Keep one AbortController, abort before each fetch, pass its signal, and swallow AbortError.
When does fetch trigger a CORS preflight, and what POST is complex?
Tests whether you know the simple-request boundary. A strong answer names the three safe POST content-types and gives a cross-origin POST with application/json plus a custom header like Authorization.
Download a large file via fetch and track ReadableStream progress
It tests streaming I/O and backpressure beyond Promise-based fetch. Acquire a reader from response.body, loop read() until done, sum chunk lengths against Content-Length, and emit progress.
Refactor a function using generics to accept any array type
It tests generic type parameters for preserving element types through a function boundary. Answer: declare T, accept T[], return T, and let the compiler infer the element type from the call site.
Explain generic constraints and provide a length-constrained generic example
Explain that extends requires properties; show T extends { length: number }; give a generic function signature.
Write a generic type-safe getProperty using keyof
Command of TypeScript generics and keyof for compile-time property access. Declare generic T, accept key as keyof T, and return T[K].
Create a generic State class with getState and setState
Write class State<T> with private T, constructor(T), getState(): T, and setState(T).
Write a generic fetchJSON<T> wrapper and explain its type safety benefits
Tests preserving type info across async boundaries via generics. Outline: write fetchJSON<T> returning Promise<T>, note response.json() is any, and show T lets callers lock in the response shape for compile-time checks.
Generic merge with an intersection return type
Use two type parameters T and U, return T & U, spread both objects; note later spread wins on key collisions and the type may not reflect that.
Implement MyReturnType<T> using conditional types and infer
This tests type-level pattern matching with conditional types and infer. Answer uses T extends (...args: any[]) => infer R ? R : never, noting infer captures return slot, non-functions yield never. Red flag: conflating runtime values with compile-time types.